حدود الانتقاء الطبيعي

Natural selection can produce striking adaptations that closely fit organisms to their environments, such as camouflage that protects butterflies from predatory birds.

However, natural selection does not always lead to ideal adaptations. Several factors constrain the outcomes of natural selection and prevent organisms from achieving a perfect match with their environment.

First, natural selection acts on heritable phenotypic variation present in a population at a given time. For example, a population of birds cannot evolve faster flight unless heritable variation affecting flight performance is present, with some birds naturally flying faster than others.

Second, natural selection is limited by developmental features inherited from distant ancestors.

For instance, birds evolved from nonflying vertebrates with four limbs. They inherited a developmental plan that made it very unlikely for wings to form from scratch. Instead, repeated changes to an existing pair of limbs allowed powered flight to evolve.

Third, selection acts on the overall phenotype of an organism, so traits are often interconnected, leading to trade-offs.

The ostrich, for example, has long legs and a large body that suit its environment. That large body, however, is too heavy for flight.